U.S. Pat. No. 5,525,094 describes a method and device allowing the resharpening by grinding of the cutting edges of knives fastened in cylindrical, rotatable knife carriers of size-reduction machines by means of releasable clamps without needing to demount the knives from their carriers. In this already known resharpening method, the grinding of the cutting edges occurs while the knife carrier is continuously rotating slowly and the radial grinding feed of the cutting edges is programmed according to the state of wear. For knife rims with cutting edges directed to the interior, as they are, for example, used to flake wood, this patent also already discloses a special sharpening device comprising a rotational disc for the knife rim placed on a machine frame as well as a cross-wise translatory moveable grinding unit and an adjustment device feeding evenly the knives toward the interior, over the cylindrical inner wall of the knife rim.
These proposals already offer the considerable technical and economical progress since the cumbersome, time-consuming and potentially injurious handling of the heavy knife kits is no longer necessary. Considering the frequent knife changes due to the rapid wear of the cutting edges, they allow a considerable reduction of the maintenance work for the concerned personnel and consequently also the downtime of the size-reduction machines. Furthermore the present state of wear in the areas adjacent to the cutting edges are recorded and taken into account when programming the grinding feed so that the protrusion of the cutting edge over the relevant inner surface of the knife rim which is decisive for the working process and the required final quality remains constant even after several resharpenings.
Yet the disadvantage of this sharpening method is, that, as a consequence of the constant rotation of the knife carrier during the grinding process, the face of the knife gets a contour which conforms with the cutting circle resulting inevitably in a clearance angle of zero. It must be admitted that a clearance angle of zero has proven to be effective when grinding a lot of products, but there are products having properties, e.g., density and coefficient of friction, which require a specially adapted clearance angle. That is the case for example for some wood species having a high density and/or a high coefficient of friction due to their high content of resin. Here a clearance angle of zero will cause too much generation of heat during the flaking process.